The present invention is related to electronic components and methods of making electronic components. More specifically, the present invention is related to electronic components and methods of making electronic components, particularly stacked leadless multi-layered ceramic capacitors with improved terminations for attachment of an external lead or lead frame or for direct lead-less attachment of the electronic component such that the component can subsequently be connected to an electronic circuit by a variety of secondary attachment materials and processes.
In general, the method of formation of a conductive termination, and materials used, is critical for reliable performance. The performance in use, when subsequently assembled in an electronic circuit, is directly related to the conductive termination. Historically, lead (Pb) based solders have been used to attach components to electronic circuit boards or to attach external leads to the electronic component. More recently, the use of hazardous substances in electrical and electronic equipment, as typified by the European RoHS legislation, has restricted the use of lead (Pb) in solder which has led the industry to seek various alternatives.
U.S. Pat. No. 6,704,189, for example, describes the use of Sn based solder with 10-30% Sb to form a contact between external leads and plated Multi-Layer Ceramic Capacitor (MLCC) components. However, the solders described have a liquidus below 270° C. By way of comparison, high-Pb solders such as Sn10/Pb88/Ag2 have a liquidus of about 290° C. It is generally recognized in the industry that a melting point at least 30° C. above any subsequent processing temperature is desirable to insure reliability of the external lead attachment. The ability to achieve high melting points has become critical since solders based on Sn, Ag and Cu, which are referred to in the art as SAC solders, are now becoming the common choice for attachment in Pb-free circuits. SAC solders have to be reflowed at higher temperatures, typically about 260° C., than the older Pb-based alternatives such as Sn63/Pb37 which had a melting point of 183° C. The contact material to the external lead, or for forming the terminal, must be capable of sustaining temperatures well above this in order not to melt, or partially melt, which causes significant reliability issues. A temperature of at least 30° C. above the melting point of the SAC solder is desired. Due to materials compatibility and higher processing temperatures involved with the semi-conductor technologies, gold/germanium, gold/silicon, and gold/tin alloys were developed to attach a die to substrates. Since the die and their mating surfaces have a low difference in thermal coefficient of expansion CTE, these alloys provided high temperature capabilities and high strengths having tensile strengths in the range of 20,000 psi and shear strengths in the range of 25,000 psi. However, these materials also require higher processing temperatures due to their higher melting points of generally above 350° C. Their high process temperature has prevented their wider use in electronics. Tin and indium have been added to combinations of Zn, Al, Ge and Mg to form higher temperature lead free solders. However, zinc and aluminum powder tend to form oxide films on the surface which are associated with poor wettability in the subsequent solders making them impractical to use. Solders with tin, zinc, cadmium, and aluminum are available but they are typically used in their eutectic alloy form because their alloys, other than eutectics, have wide plastic ranges of 50-175° C. limiting their use to very specific applications outside of electronics. Cadmium, zinc, and silver alloy solders are good for soldering aluminum. Once the liquidus temperatures move above 450° C. the solders are referred to as brazing solders which are typically used in structural applications rather than electrical applications. Methods of forming Pb-free, high temperature bonds to capacitors that retain their integrity above 260° C. and are economical to manufacture have therefore yet to be realized.
The following patents describe the materials and processes of TLPS with respect to forming a conductive bonds. U.S. Pat. No. 5,038,996 describes coating two mating surfaces one with Sn and the other with Pb and forming a joint by raising the process temperature to a temperature of about 183° C. which is slightly below the melting point of Sn. Transient Liquid Phase Sintering (TLPS) formulations disclosed in U.S. Pat. No. 5,853,622 combine TLPS materials with cross linking polymers to create a conductive adhesive having improved electrical conductivity as a result of intermetallic interfaces between the metal surfaces created by TLPS process. The spraying of two mating surfaces, with one surface having a low temperature melting material and the mating surface having a compatible higher melting temperature material forms a joint when heating to the melting point of the lower temperature material as discussed in U.S. Pat. No. 5,964,395.
U.S. Pat. No. 5,221,038 describes the use of SnBi or SnIn for soldering discrete components such as resistors and the like to printed circuit boards using the TLPS process. The use of Ag/SnBi coated to two mating surfaces to mount electronic modules to substrates was disclosed in U.S. Pat. No. 6,241,145. U.S. Pat. Publ. No. 2002/0092895 discusses the deposition of materials on two mating surfaces, a substrate and the surface of the bumps on a flip chip, elevated to a temperature to cause diffusion between the materials to create a TLPS compatible alloy. U.S. Pat. Publ. No. 2006/0151871 describes the use of TLPS in forming packages containing SiC or other semiconductor devices bonded to other components or conductive surfaces. U.S. Pat. Publ. No. 2007/0152026 describes the placement of TLPS compatible materials on mating surfaces followed by reflowing the lower melting point material and then isothermal aging to complete the diffusion process where the two devices to be joined are a micro-electromechanical system (MEMS) device to a microelectronic circuit. U.S. Pat. No. 7,023,089 describes the use of TLPS to bond heat spreaders made from copper, black diamond, or black diamond copper composite to silicon die. These patents and applications describe the processing of TLPS to bond components to circuit boards but do not contain any teaching regarding their use to form terminations on electronic components or in the attachment of components to lead frames.
In a more recent development U.S. Pat. Publ. No. 2009/0296311 describes a high temperature diffusion bonding process that welds the lead to the inner electrodes of a multi-layer ceramic component. TLPS materials are plated on the faces of mating surfaces to be joined together by introducing heat to initiate the diffusion process. In this case, intimate mutual contact across the surfaces is required between the component and lead frame to facilitate the diffusion. This limits the application to the joining of surfaces that can form an intimate line of contact and this application cannot accommodate components of differing length connected to the lead frame. Furthermore, high temperatures in the range of 700 to 900° C. are described to achieve a welded bond. These high formation temperatures require careful process design, such as preheating stages, to avoid thermal shock damage to the multi-layer ceramic components and even then this may not be suitable for all materials.
Other Pb free attachment technologies are described in the art yet none are adequate.
Solder is an alloy consisting of two or more metals that have only one melting point, which is always lower than that of the metal having the highest melting point and generally has a melting point of less than about 310° C. depending on the alloy. Solder can be reworked, meaning it can be reflowed multiple times, thus providing a means to remove and replace defective components. Solders also make metallurgical bonds by forming intermetallic interfaces between the surfaces they are joining. As solders wet to their adjoining surfaces, they actually flow outward and spread across the surface areas to be joined.
MLCC's are widely used in a variety of applications. Most typically an MLCC, or a stack of MLCC's, is mounted to a circuit board as a discrete component. A particular problem associated with MLCC's is their propensity to crack when subjected to stress such as bending of the circuit board. To avoid these stress fractures the MLCC's are mounted between lead frames, such as one of each polarity, and the lead frames are then attached to the circuit board by soldering and the like. The lead frames have been considered in the art to be a necessity and much effort has been spent designing lead frames capable of withstanding the stress associated with board flexure without imparting the stress on the MLCC. The lead frame design and material is particularly difficult due to the differences in coefficient of thermal expansion and the desire to minimize equivalent series resistance (ESR), inductance and other parasitics. In spite of the desire to eliminate the lead frame those of skill in the art have not been able to do so since any flexure of the circuit board transfers directly to the MLCC virtually insuring damage to the MLCC.
In spite of the ongoing, and intensive effort, the art still lacks an adequate capacitor. There is an ongoing need for lead connections with improved reliability for high temperature applications, especially lead (Pb) free.